Ever more stringent exhaust-gas laws together with the pressure to reduce fuel consumption necessitate new concepts both for the internal combustion engine and also for exhaust-gas purification. This also demands new concepts for the control and monitoring of exhaust-gas purification systems.
For example, in the case of the stoichiometrically operated applied-ignition engine (so-called “λ=1 engine”), the air/fuel ratio λ (also referred to as the air number) of the untreated exhaust gas is detected by means of a first λ probe. In the event of a control deviation from the setpoint value λ=1, the air/fuel ratio is then corrected. It is necessary for approximately λ=1 to be adhered to on average over time. Owing to the oxygen storage capacity of the so-called “three-way catalytic converter” arranged downstream of the first λ probe, optimum conversion takes place for as long as the catalytic converter is still in a good state. With decreasing catalytic converter quality, which is manifested in a reduction of the conversion rate and a rise in the light-off temperature, the capability of the catalytic converter to store oxygen also decreases. A second λ probe arranged downstream of the catalytic converter can detect this. For such an indirect process, in which the state of the oxygen-storing catalytic converter is inferred from the signals of the two λ probes, highly complex modelling is necessary, which necessitates in particular an engine operating state model, see for example J. Riegel et al., “Exhaust gas sensors for automotive emission control”, Solid State Ionics 152-153 (2002), 783-800.
This is addressed by processes which determine the operating state and the quality of a catalytic converter which stores gases such as for example oxygen. In particular, it is possible with said processes to determine the extent to which the oxygen store of the catalytic converter is filled or where the oxygen loading front in the catalytic converter is situated, as shown for example in R. Moos, M. Wedemann, M. Spörl, S. Reiβ, G. Fischerauer, “Direct Catalyst Monitoring by Electrical Means: An Overview on Promising Novel Principles”, Topics in Catalysis, 52 (2009), 2035-2040. Of particularly simple design here are so-called high-frequency-based systems such as are described for example in DE102008012050 or in DE10358495.
In said processes, an electromagnetic microwave resonance is excited in the interior space of the catalytic converter housing formed as a cavity resonator, and the shift of the resonance frequency and/or quality is observed. The change in the resonance frequency is taken for example as a measure for the oxygen loading of the storage material of the catalytic converter. DE102008012050 proposes regulation based on this. When a predefinable value of the resonance frequency is attained, a regeneration is carried out. As already indicated in DE102008012050 and also presented therein for example on the basis of FIG. 8, the temperature of the system plays a significant role because the temperature causes a shift of the resonance frequencies. Even with knowledge of the catalytic converter temperature, however, it is not possible by means of the measurement system to directly derive important variables such as for example the oxygen loading. The background to this is that firstly the resonance frequencies are dependent on the geometry, and secondly the temperature dependency of the loading is unknown. Specifically the geometry aspect also prevents a high-frequency-based measurement system installed in the exhaust tract from being able to be used without further calibration, because the component variance with regard to geometric dimensions and with regard to a layer thickness variance of the coating and with regard to the reproducibility of the electrical properties of the ceramic substrate (of the honeycomb body) can lead to variance in the resonance frequencies, which variance is of the order of magnitude of the measurement effect.